1. Field of the Invention
The present invention relates generally to concentric annular ring lens designs for astigmatic patients, and more particularly pertains to such lens designs which reduce the sensitivity of the patient to toric axis misalignment, thus reducing the required number of stock keeping units in inventory (the total number of different prescriptions which are maintained in stock and can be prescribed) for a toric product.
The present invention provides a novel approach for correcting ametropias such as astigmatism, hyperopia and myopia, with a primary correction for astigmatism.
2. Discussion of the Prior Art
Currently, toric lenses are manufactured in the prior art with the following design features:
a. a toric curve on the front or back surface of the lens; PA1 b. prism ballast and slab-off features on the front surface of the lens; PA1 c. the non-toric surface is spherical.
These prior art designs correct astigmatism adequately only if the axis of the cylindrical power is accurately aligned with respect to the axis of the astigmatic cornea. A misalignment of the axes (greater than 10.degree.) results in a substantial loss of visual acuity. The primary source of this misalignment in soft hydrogel contact lenses is poor rotational stability.
In conventional prior art toric lens designs, a single toric surface comprising a major and minor axis is placed in the optical portion of either the front or back surface of the lens. In addition, the axes of the toric lens are usually stabilized in relation to the patient's corneal axes through the use of either a prism ballasted/slab-off feature or a double slab-off feature placed on the front surface of the lens. These features tend to increase the thickness of the resultant lenses and compromise wearing comfort and physiological acceptability.
Moreover, conventional toric lens designs require a large number of stock keeping units in inventory (the total number of different prescriptions which are maintained in stock and can be prescribed) in order to fit a broad astigmatic patient base. For example, current Frequent Replacement Toric lens products are available in 800 stock keeping units per base curve in inventory (40 spherical powers.times.2 cylindrical powers.times.10 different cylindrical axis placements). Such a large number of stock keeping units per base curve in inventory is uneconomical to produce and maintain, particularly for a disposable modality product. The required large number of stock keeping units in inventory arises primarily from the need to provide 10 or more different cylindrical axes placements. Furthermore, any significant misalignment of the cylindrical axis with respect to the cylindrical axes of the eye normally results in a significant loss of visual acuity.
In an attempt to reduce the required number of cylindrical axis placements in stock keeping units, Australian Published Patent Application WO 93/03409 combines aspherical surfaces with toric surfaces to accommodate axial misalignment through the increased depth-of-focus provided by aspheres. The use of an aspheric surface enhances the depth-of-field of toric lenses and minimizes the effect of rotational misalignment of the toric lenses. Complex optics such as diffraction optics using echelets or birefringence optics are also disclosed by this published patent application. One disadvantage in using this prior art approach is the difficulty in manufacturing and controlling such complex optics and aspheres. Additionally with aspheric optics, patient anatomical variations have been shown to produce compromised visual acuity. In summary, this prior art approach is undesirable because of the high level of visual unpredictability of aspheric optics on the real world patient base, and because of the difficulty in manufacturing and controlling aspheric and other complex optics.